Distance measuring device

Abstract

To provide a configuration which can correctly measure a distance to a measurement object under such an environment that space floating fine particles such as fog exist.SOLUTION: A signal processing part 32 in a light reception unit 30 detects falling timing Tb at which a response waveform generated according to a change in the number (response element number N) of single photon avalanche diodes (SPADs) in response to reflection light in the prescribed number of SPADs becomes equal to or less than a prescribed threshold Nth after exceeding the prescribed threshold Nth for each pixel. A measurement unit 40 performing distance measurement in a measurement target region on the basis of a time difference ΔT between light projection timing of a light projection unit 20 and light reception timing obtained for each pixel in the light reception unit 30 obtains the light reception timing so as to be the timing obtained by subtracting the dead time ΔTd of the SPAD from the falling timing Tb detected by the signal processing part 32 when being affected by fog or the like.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a distance measurement device that measures the distance to a measurement object by utilizing reflected light received from the measurement object. [Background technology]

[0002] Conventionally, optical distance measurement devices have been used to measure the distance to an object using reflected light received from the object within a measurement area. These devices use the timing at which reflected light from light irradiated onto the object is detected, i.e., the time of flight (TOF) of light traveling to and from the object.

[0003] One example of a technology related to such optical distance measurement devices is the optical distance measurement device disclosed in Patent Document 1 below. This optical distance measurement device employs a TOF measurement photodetector and a noise measurement photodetector, each of which has a vertically parallel arrangement of SiPMs (Silicon Photo Multipliers), each of which is configured as a photodetector by arranging a predetermined number of SPADs (Single Photon Avalanche Diodes) in an array. The TOF measurement photodetector generates a histogram (response waveform) by measuring the elapsed time from the laser emission time when the number of SPADs in a high state (High) is equal to or greater than a predetermined number. If the peak value in this histogram is equal to or greater than a predetermined threshold, the time corresponding to the peak position is output as the TOF from the histogram circuit. A previously created noise average-threshold conversion table is referenced, and a boundary level corresponding to the noise average value output by the detection circuit of the noise measurement photodetector is set as the predetermined threshold of the histogram circuit, thereby maintaining the false detection rate of the reflected light signal at a predetermined value according to the noise level. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-134224 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as described above, in a configuration in which one pixel of the light receiving unit is composed of a predetermined number of SPADs and TOF measurement is performed using a histogram (response waveform) generated in accordance with the number of SPADs responding to reflected light from an object (measurement object) within the measurement target area, accurate distance measurement may not be possible in an environment where airborne particles such as fog are present. If reflected light generated by fog around the distance measurement device is received immediately after a predetermined light projection pulse is projected onto the measurement target area, the number of responding SPADs increases rapidly immediately after the projection, causing the histogram to saturate. In such a case, in a configuration in which the timing at which the histogram rises above a predetermined threshold is used to measure the distance to the measurement object, the rapid increase in the histogram immediately after the projection exceeds the predetermined threshold, resulting in an erroneous measurement if the measurement object is located close by.

[0006] The present invention has been made to solve the above-mentioned problems, and its purpose is to provide a configuration that can accurately measure the distance to a measurement object even in an environment where airborne particles such as fog are present. [Means for solving the problem]

[0007] In order to achieve the above object, the invention described in claim 1 of the claims is as follows: a light projection unit (20) that projects a predetermined light projection pulse onto a predetermined measurement target area; a light receiving section (30) that is configured by arranging a plurality of light receiving elements each having a predetermined number constituting one pixel, and that receives light that is reflected by an object within the measurement target area from the light projection pulse; a measurement unit (40) that measures distance within the measurement target area based on the time difference between the light projection timing of the light projection unit and the light reception timing obtained for each pixel in the light reception unit; A distance measurement device comprising: the light receiving unit includes a signal processing unit (32) that detects, for each pixel, a falling timing (Tb) at which a response waveform generated in response to a change in the number (N) of light receiving elements that respond to the reflected light, exceeds a predetermined threshold (Nth) and then falls below the predetermined threshold; A distance measurement device characterized in that the measurement unit determines the light receiving timing to be the timing obtained by subtracting the dead time (ΔTd) of the light receiving element from the falling timing detected by the signal processing unit. The symbols in parentheses above indicate the correspondence with the specific means described in the embodiments to be described later. [Effects of the Invention]

[0008] In the invention of claim 1, the signal processing unit included in the light receiving unit detects, for each pixel, a falling timing at which a response waveform generated in response to a change in the number of light receiving elements that respond to reflected light, among a predetermined number of light receiving elements, exceeds a predetermined threshold and then falls below the predetermined threshold.The measurement unit, which measures the distance within the measurement target area based on the time difference between the light projection timing of the light projecting unit and the light reception timing calculated for each pixel in the light receiving unit, calculates the light reception timing to be the falling timing detected by the signal processing unit minus the dead time of the light receiving elements.

[0009] When a light projection pulse is projected toward a measurement object in an environment where airborne particles such as fog are present, no reflected light occurs at distances farther than the measurement object. Therefore, the response waveform is generated such that it rises sharply immediately after projection due to the presence of fog, and then falls sharply depending on the distance to the measurement object. That is, while the rising edge of the response waveform is affected by fog, the falling edge of the response waveform is not. In addition, in a response waveform generated in response to reception of reflected light from a measurement object in the absence of fog, the time difference between the rising edge exceeding a predetermined threshold and the subsequent falling edge below the predetermined threshold corresponds to the dead time of the light receiving element. Therefore, by subtracting the dead time of the light receiving element from the falling edge to estimate the rising edge, and then determining the light reception timing to match this estimated rising edge, distance measurement can be performed using the falling edge that is not affected by fog. Therefore, the distance to the measurement object can be accurately measured even in an environment where airborne particles such as fog are present.

[0010] In the invention of claim 2, the light receiving element is a SPAD (Single Photon Avalanche Diode), and the light receiving unit has a passive quench circuit for recovering the voltage applied to the SPAD, which has dropped from a predetermined voltage value in response to a breakdown phenomenon caused by light reception, to the predetermined voltage value when the voltage stops dropping during light reception and light reception ceases.The measurement unit determines the light reception timing to be the timing obtained by subtracting a dead time and a predetermined light projection pulse width from the falling timing detected by the signal processing unit.

[0011] In a light-receiving unit having a passive quench circuit that restores the applied voltage to a SPAD, which has dropped from a predetermined voltage value in response to a breakdown phenomenon caused by light reception, to the predetermined voltage value when the voltage stops decreasing during light reception and no longer receives light, the applied voltage does not recover while the reflected light of the light projection pulse is received. That is, the longer the light projection pulse width of the light projection pulse, the later the recovery start timing at which the applied voltage begins to recover, resulting in a longer time difference between the rising and falling edges. Therefore, by estimating the rising edge timing by subtracting the dead time of the light-receiving element and the light projection pulse width from the falling edge timing, the deviation in the recovery start timing of the applied voltage due to the use of a passive quench circuit can be suppressed. Therefore, even when a passive quench circuit is used, the degradation in the estimation accuracy of the rising edge timing and the light reception timing due to the deviation in the recovery start timing of the applied voltage can be suppressed.

[0012] In the invention of claim 3, when the signal processing unit detects two or more falling edge timings for one pixel, the measurement unit determines the light receiving timing based on the latest falling edge timing.

[0013] As a result, even if two or more falling edge timings are detected due to a thin concentration of fog or the like, reflected light does not occur at a distance greater than the measurement object, so by determining the light reception timing based on the latest falling edge timing, erroneous measurements caused by the concentration of fog or the like can be suppressed.

[0014] In the invention of claim 4, the signal processing unit detects, for each pixel, the rising timing (detected rising timing) at which the response waveform exceeds a predetermined threshold value, together with the falling timing. The measurement unit determines the light receiving timing to be the timing obtained by subtracting the dead time of the light receiving element from the falling timing when the time difference between the rising timing and the timing obtained by subtracting the dead time of the light receiving element from the falling timing is equal to or greater than the predetermined time threshold, and determines the light receiving timing to be the rising timing when the time difference between the rising timing and the timing obtained by subtracting the dead time of the light receiving element from the falling timing is less than the predetermined time threshold.

[0015] In an environment where there is no airborne particle such as fog, the time difference between the timing obtained by subtracting the dead time of the light receiving element from the fall timing (estimated rise timing) and the detected rise timing becomes small. Furthermore, in an environment where there is no airborne particle such as fog, the light reception timing calculated to be the detected rise timing has higher distance measurement accuracy than the light reception timing calculated to be the estimated rise timing. Therefore, if the time difference is equal to or greater than a predetermined time threshold, it is determined that the environment is one in which airborne particle such as fog exists, and the light reception timing is calculated to be the estimated rise timing. If the time difference is less than the predetermined time threshold, it is determined that the environment is one in which airborne particle such as fog does not exist, and the light reception timing is calculated to be the detected rise timing. This allows the distance measurement accuracy for measuring the distance to the measurement object to be improved by changing the way the light reception timing is calculated depending on the presence or absence of fog.

[0016] In the invention of claim 5, the measurement unit does not measure the distance to a pixel where the gradient before and after the falling timing of the response waveform is equal to or less than a predetermined gradient threshold value.

[0017] In an environment where fog or the like is present, if a measurement object is present at the destination of the light projection pulse, the response waveform is generated so that it drops sharply according to the distance to the measurement object, and if no measurement object is present at the destination of the light projection pulse, the response waveform is generated so that it falls gradually. Therefore, if the slope before and after the falling timing of the response waveform is equal to or less than the predetermined slope threshold, it is determined that no measurement object is present at the destination of the light projection pulse and therefore the response waveform is generated so that it falls gradually, and distance measurement is not performed for that pixel, thereby making it possible to suppress erroneous measurements caused by fog or the like.

[0018] In the invention of claim 6, the light receiving element is a SPAD (Single Photon Avalanche Diode), and the light receiving unit has an active quench circuit for restoring the voltage applied to the SPAD, which has dropped from a predetermined voltage value in response to a breakdown phenomenon caused by light reception, to the predetermined voltage value after a predetermined time even during light reception.The light projecting unit then projects a light projection pulse so that the amount of light gradually increases.

[0019] In an active quench circuit, which restores the applied voltage even while receiving light, each SPAD returns to a state where it can respond to reflected light in a shorter time than a passive quench circuit. Therefore, when strong reflected light is received from fog or other sources at close range during dense fog, an oscillation phenomenon may occur in which the response waveforms of almost all SPADs repeatedly exceed a predetermined threshold at the same time. For this reason, by projecting a light projection pulse so that the light intensity gradually increases, the timing at which the response waveforms of each SPAD exceed the predetermined threshold can be dispersed compared to when a rectangular light projection pulse is projected, thereby suppressing the above-mentioned oscillation phenomenon. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a block diagram showing a schematic configuration of a distance measurement device according to a first embodiment. [Figure 2] FIG. 1 is an explanatory diagram illustrating distance measurement using a TOF measurement method. [Figure 3]FIG. 10 is an explanatory diagram illustrating a change in the voltage applied to a SPAD using an active quench circuit. [Figure 4] This is an explanatory diagram showing the response waveforms generated according to changes in the number of response elements. Figure 4(A) shows the measurement results in an environment where there are no airborne particles such as fog on the way to the measurement object, and Figure 4(B) shows the measurement results in an environment where there are airborne particles such as fog on the way to the measurement object. [Figure 5] 4 is a flowchart illustrating the flow of measurement processing and the like performed by a measurement unit in the first embodiment. [Figure 6] 6A and 6B are explanatory diagrams illustrating a case where multiple fall timings occur as a modified example of the first embodiment, where FIG. 6A shows a case where the density of fog, etc. is thinner than that in FIG. 4A, and FIG. 6B shows a case where the density of fog, etc. is thinner than that in FIG. 6A. [Figure 7] 10 is an explanatory diagram illustrating a change in the voltage applied to a SPAD that uses a passive quench circuit employed in the light receiving section in the second embodiment. FIG. [Figure 8] 10A and 10B are explanatory diagrams illustrating a response waveform generated when there is no measurement object at the destination of the light projection pulse in an environment where fog or the like is present. [Figure 9] 11 is a flowchart illustrating the flow of a measurement process and the like performed by a measurement unit in the third embodiment. [Figure 10] FIG. 10(A) is an explanatory diagram illustrating an example in which the supply current supplied to each light-projecting element is increased linearly in order to project light-projection pulses whose light intensity gradually increases in the fourth embodiment, and FIG. 10(B) is an explanatory diagram illustrating an example in which the supply current supplied to each light-projecting element is increased curvedly in order to project light-projection pulses whose light intensity gradually increases in a modified example of the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] [First embodiment] A first embodiment of a distance measurement device according to the present invention will be described below with reference to the drawings. The distance measurement device 10 according to this embodiment is a measurement device such as a LiDAR that measures the distance to an object within a predetermined measurement target area using a TOF measurement method. As shown in Fig. 1, this distance measurement device 10 is configured to include a control unit 11 that handles overall control, a light projecting unit 20 that projects a predetermined light projecting pulse onto the predetermined measurement target area, a light receiving unit 30 that receives light reflected by an object within the measurement target area, and a measurement unit 40 that measures the distance to the object within the measurement target area based on the light receiving pulse received by the light receiving unit 30.

[0022] In a distance measurement device 10 that employs the TOF measurement method, as shown in Figure 2, distance measurement is performed in pixel units in the light receiving unit 30 according to the time difference ΔT between the timing at which a light projection pulse is projected by the light projecting unit 20 and the timing at which the light reflected by the measurement object R in response to that light projection pulse is received by the light receiving unit 30 as a light receiving pulse.

[0023] The control unit 11 is mainly composed of a microcomputer and has a CPU, a system bus, an input / output interface, etc., and constitutes an information processing device together with a storage unit consisting of a ROM, a RAM, a non-volatile memory, etc. The storage unit stores in advance programs related to the control of the light projecting unit 20 and the measuring unit 40, programs for executing control processing using the measurement results by the measuring unit 40, etc., so that they can be executed by the control unit 11.

[0024] The light-projecting unit 20 includes a VCSEL array 21 and a light source control unit 22 that controls each light-projecting element of the VCSEL array 21 in response to a light-projection command from the control unit 11. The VCSEL array 21 is configured such that a plurality of vertical-cavity surface-emitting lasers, which are light-projecting elements, are arranged in an array, and is controlled by the light source control unit 22 so as to be able to project a rectangular light-projection pulse at a predetermined timing for each light-projecting element.

[0025] The light receiving unit 30 includes a SPAD array 31 and a signal processing unit 32 that processes output signals from the SPAD array 31 and outputs the processed signals to the measurement unit 40. The SPAD array 31 has a plurality of SPADs, which are employed as light receiving elements having high response characteristics and allowing for a small pixel size, arranged in an array similar to the light projecting elements of the VCSEL array 21, and a predetermined number (e.g., 18) of SPADs form one pixel. In this way, the SPAD array 31, which is made up of a plurality of arrayed SPADs of a predetermined number that form one pixel, is arranged so that light projecting pulses projected from the light projecting unit 20 are reflected by objects within the measurement target area and are received by each SPAD as light receiving pulses.

[0026] The light-receiving unit 30 also includes an active quench circuit that quickly restores each SPAD to a state where it can respond to a received light pulse (reflected light). As shown in Figure 3, the active quench circuit applies a reverse bias voltage Va to each SPAD. When a voltage V applied to a SPAD drops from Va due to a breakdown caused by light reception, the active quench circuit forcibly restores the voltage V to Va after a predetermined time, even during light reception. The main difference between this active quench circuit and a passive quench circuit (described later) is that the active quench circuit does not use a quench resistor, which is a passive element, but instead uses an active element such as a transistor to prevent the voltage V applied to the SPAD from being restored until a certain delay time has elapsed after a breakdown occurs, and then quickly restores the voltage V to Va immediately after the delay time has elapsed. (Note that a hybrid quench circuit incorporating a quench resistor may also be used to improve the performance of the active quench circuit.) That is, the active quench circuit restores the SPAD to a responsive state within a constant SPAD dead time ΔTd, regardless of the light projection pulse width (the time during which the light projection pulse is projected). An example of an active quench circuit that functions in this manner is the active circuit disclosed in Japanese Patent Application Laid-Open No. 2020-134224. The reverse bias voltage Va may be an example of a "predetermined voltage value."

[0027] The signal processing unit 32 is configured to process the signals output by each SPAD in response to receiving a light-receiving pulse, and output the signals to the measurement unit 40. Specifically, the signal processing unit 32 performs a light-receiving timing information output process in which, for each sampling time, it generates a response waveform (histogram) in response to changes in the number of SPADs (hereinafter also referred to as the number of response elements N) that responded to the light-receiving pulse among a predetermined number of SPADs that make up one pixel, and obtains information on the light-receiving timing used in TOF measurement (hereinafter also referred to as light-receiving timing information) for each pixel from this response waveform and outputs the information to the measurement unit 40.

[0028] In the light reception timing information output process of this embodiment, first, the timing when the applied voltage V of the SPAD after the light projection pulse is projected becomes equal to or less than a predetermined voltage threshold Vth (see FIG. 3) is defined as the response timing of the SPAD, and a response waveform corresponding to the change in the number of response elements N is generated for each pixel. In an environment where there are no airborne particles such as fog between the measurement object R and the measurement object R, a response waveform such as that shown in FIG. 4A is generated. Then, the rising timing Ta1 at which the response waveform exceeds a predetermined threshold (determination threshold) Nth and the falling timing Tb at which the response waveform subsequently falls below the predetermined threshold Nth are detected for each pixel. Furthermore, the rising timing Ta2 is estimated by subtracting the dead time ΔTd of the SPAD from the detected falling timing Tb, and the estimated rising timing Ta2 and the detected rising timing Ta1 and falling timing Tb are output to the measurement unit 40.

[0029] The reason for estimating the rising timing Ta2 from the falling timing Tb in this way will be explained below. In an environment where there are airborne particles such as fog, pollen, and PM2.5 (fine particulate matter), if reflected light (including continuous scattering by particles) caused by fog around the distance measurement device 10 is received immediately after a predetermined light projection pulse is projected onto the measurement target area, the number of responding SPADs will increase rapidly immediately after the light projection, reaching a maximum value Nmax, causing the response waveform to saturate, as shown in Figure 4(B). Note that in Figure 4(B), the response waveform (corresponding to Figure 4(A)) generated when reflected light is received from the same measurement object R in an environment where there is no fog or the like is shown by a dashed line.

[0030] In such a case, if the configuration measures the distance to the measurement object R using the rising timing Ta1 when the response waveform exceeds a predetermined threshold Nth as the timing when reflected light from the measurement object R is received, the response waveform, which increases sharply immediately after light projection, will exceed the above-mentioned predetermined threshold Nth, resulting in an erroneous measurement that the measurement object R is located at a close distance.

[0031] On the other hand, when a light projection pulse is projected onto measurement object R in an environment where airborne particles such as fog are present, no reflected light is generated at a distance farther than measurement object R, and therefore, as illustrated in Fig. 4(B), the response waveform is generated so that it rises sharply immediately after projection due to the presence of fog, etc., reaches a maximum value Nmax, and then falls sharply according to the distance to measurement object R. In other words, even if the rise timing Ta1 of the response waveform is affected by fog, etc., the fall timing Tb of the response waveform is not affected by fog, etc.

[0032] In the response waveform generated in response to the reception of reflected light from the measurement object R when there is no fog or fog, the time difference between the rising edge Ta1, when the signal exceeds a predetermined threshold Nth, and the falling edge Tb, when the signal subsequently falls below the predetermined threshold Nth, corresponds to the dead time ΔTd of the SPAD. Strictly speaking, the area on the object corresponding to one pixel is small, and each SPAD element constituting one pixel receives reflected light at essentially the same time. Each SPAD element that responds at the same time continues to respond for the dead time ΔTd. Because the received light waveform in this example is generated using the sum of the output values ​​(binary signal (0,1)) of each SPAD element constituting one pixel, the half-width of the signal waveform (Tb - Ta1) = dead time ΔTd can be derived. Note that this relationship may not hold depending on the predetermined threshold Nth of the response waveform, but the impact is not significant. It is also possible to correct the above relational expression depending on the level of Nth. For example, if the level of Nth is lower than half the value of the signal light, then Tb-Ta1>ΔTd, and if the level of Nth is high, then Tb-Ta1<ΔTd. Therefore, multiplying by a predetermined coefficient according to the level of Nth can improve distance accuracy. In this embodiment, the median value of the dead time of each SPAD is used as the dead time ΔTd of the SPAD.

[0033] Therefore, in an environment where fog or the like is present, the rise timing Ta2 is estimated by subtracting the SPAD dead time ΔTd from the fall timing Tb (see Figure 4(B)), and the light reception timing is determined to be this estimated rise timing Ta2, thereby making it possible to perform distance measurement using the fall timing Tb that is not affected by fog or the like.

[0034] The measurement unit 40 receives light reception timing information (detected rising timing Ta1 and falling timing Tb and estimated rising timing Ta2) calculated for each pixel as described above from the signal processing unit 32 of the light receiving unit 30, and also receives information regarding the timing of the light projection instruction issued to the light projecting unit 20 from the control unit 11 (hereinafter also referred to as light projection timing information), thereby performing measurement processing to measure the distance to an object within a specified measurement target area.

[0035] Hereinafter, the measurement process and the like performed by the measurement unit 40 in this embodiment will be described in detail with reference to the flowchart shown in FIG. When measurement processing is started in the measurement unit 40 in response to a predetermined operation or the like, each light-projecting element of the VCSEL array 21 of the light-projecting unit 20 becomes ready to emit light, and each SPAD of the SPAD array 31 of the light-receiving unit 30 becomes ready to receive light. Then, in response to a light emission instruction from the control unit 11, a light projection pulse is projected (emitted) from the light-projecting unit 20 (S101 in FIG. 5).

[0036] Next, light-receiving timing information is acquired in step S103. In this process, the light-receiving timing information (detected rising timing Ta1 and falling timing Tb and estimated rising timing Ta2) calculated for each pixel by the light-receiving timing information output process performed by the signal processing unit 32 of the light-receiving unit 30 as described above is acquired from the signal processing unit 32.

[0037] Next, it is determined for each pixel whether the reflected light from the measurement object is received as a light-receiving pulse, and the distance to the measurement object is calculated for each pixel that receives light. Specifically, first, the first pixel is set as the measurement target pixel, and in the determination process shown in step S105, it is determined whether the measurement target pixel has received a light-receiving pulse. Here, if the response waveform generated for the measurement target pixel does not exceed the above-mentioned predetermined threshold value Nth, it is determined that the measurement target pixel has not received a light-receiving pulse (No in S105), and the measurement target pixel change process shown in step S107 is performed. In this process, the measurement target pixel is changed to the next pixel, and the determination process of step S105 is performed for this changed next measurement target pixel.

[0038] If the response waveform generated by the changed measurement target pixel exceeds the above-mentioned predetermined threshold value Nth, it is determined that the measurement target pixel has received a light-receiving pulse (Yes in S105). In this case, in the determination process of step S109, it is determined whether or not the detected rising timing Ta1 is affected by fog or the like.

[0039] Here, since the number of response elements N does not increase suddenly immediately after light projection, if the time difference between the detected rising timing Ta1 and the estimated rising timing Ta2 is less than a predetermined time threshold, it is determined that the detected rising timing Ta1 is not affected by fog, etc. (No in S109: fog determination flag OFF).

[0040] When it is determined that there is no influence of fog or the like in this way, a distance calculation process shown in step S111 is performed. In this process, the detected rising timing Ta1 is set as the light receiving timing, and the distance to the measurement object at the measurement target pixel is calculated according to the time difference ΔT between this rising timing Ta1 and the light projection timing (light projection timing information) acquired from the control unit 11.

[0041] On the other hand, as described above, in an environment where fog or the like is present, the number of response elements N increases sharply immediately after light is projected. Therefore, if the difference between the rising timing Ta1 and the estimated rising timing Ta2 is equal to or greater than the above-mentioned predetermined time threshold, it is determined that the detected rising timing Ta1 is affected by fog or the like (Yes in S109: fog determination flag ON).

[0042] When it is determined that there is an influence of fog or the like in this way, a distance calculation process shown in step S113 is performed. In this process, the estimated rising timing Ta2 (the timing obtained by subtracting the dead time ΔTd of the SPAD from the falling timing Tb) is used as the light receiving timing, and the distance to the measurement object at the measurement target pixel is calculated according to the time difference ΔT between this rising timing Ta2 and the light projection timing (light projection timing information) acquired from the control unit 11.

[0043] After the distance at the measurement target pixel is calculated as described above, if the light reception determination process has not been performed for all pixels (No in S115), the processes from step S107 onwards are performed. Then, when the distance to each measurement object has been calculated for all pixels that have received a light reception pulse (Yes in S115), the measurement result output process shown in step S117 is performed, and the results are output to a higher-level device or stored in the memory of control unit 11, thereby completing the main measurement process for one sampling (one light projection).

[0044] As described above, in the distance measurement device 10 according to this embodiment, the signal processing unit 32 included in the light receiving unit 30 detects, for each pixel, the falling timing Tb at which a response waveform generated in response to a change in the number of SPADs (number of response elements N) that respond to reflected light among a predetermined number of SPADs exceeds a predetermined threshold Nth and then falls below the predetermined threshold Nth. Then, the measurement unit 40, which performs distance measurement within the measurement target area based on the time difference ΔT between the light projection timing of the light projector 20 and the light reception timing determined for each pixel in the light receiving unit 30, determines, when affected by fog or the like, the light reception timing to be the falling timing Tb detected by the signal processing unit 32 minus the dead time ΔTd of the SPADs.

[0045] In this way, when there is an influence of fog, etc., the rise timing Ta2 is estimated by subtracting the SPAD dead time ΔTd from the fall timing Tb, and the light receiving timing is determined so that it coincides with this estimated rise timing Ta2, thereby making it possible to perform distance measurement using the fall timing Tb that is not influenced by fog, etc. Therefore, the distance to the measurement object can be accurately measured even in an environment where there are airborne particles such as fog.

[0046] In particular, the signal processing unit 32 detects, for each pixel, the rising timing (detected rising timing Ta1) at which the response waveform exceeds a predetermined threshold Nth, together with the falling timing Tb. In the measurement process performed by the measurement unit 40, if the time difference between the estimated rising timing Ta2 (the timing obtained by subtracting the SPAD dead time ΔTd from the falling timing Tb) and the detected rising timing Ta1 is equal to or greater than a predetermined time threshold (Yes in S109), the light-receiving timing is determined to be the estimated rising timing Ta2, and if the time difference between the estimated rising timing Ta2 and the detected rising timing Ta1 is less than the predetermined time threshold (No in S109), the light-receiving timing is determined to be the detected rising timing Ta1.

[0047] In an environment where there is no airborne particle such as fog, the time difference between the estimated rising timing Ta2 and the detected rising timing Ta1 is small. Furthermore, in an environment where there is no airborne particle such as fog, the light reception timing calculated to coincide with the detected rising timing Ta1 provides higher distance measurement accuracy than the light reception timing calculated to coincide with the estimated rising timing Ta2. This is because, for both the light projection pulse and the light reception pulse, the rising edge has a steeper characteristic than the falling edge, and therefore, using the rising edge improves distance measurement accuracy.

[0048] Therefore, if the time difference is equal to or greater than a predetermined time threshold, it is determined that the environment is one in which airborne particles such as fog exist, and the light-receiving timing is calculated to be the estimated rising timing Ta2, whereas if the time difference is less than the predetermined time threshold, it is determined that the environment is one in which airborne particles such as fog do not exist, and the light-receiving timing is calculated to be the detected rising timing Ta1. In this way, by changing the way the light-receiving timing is calculated depending on the presence or absence of fog, it is possible to improve the distance measurement accuracy for measuring the distance to the measurement object.

[0049] As a modification of this embodiment, when two or more falling timings Tb are detected for one pixel in an environment where fog or the like is present, the light reception timing may be determined based on the latest falling timing Tb. For example, when the density of fog or the like is lighter than that of the environment in FIG. 4B, as illustrated in FIG. 6A, the number of response elements N may become equal to or less than a predetermined threshold Nth in front of the measurement object R, and two or more timings at which the number of response elements N becomes equal to or less than the predetermined threshold Nth may occur. Furthermore, even when the density of fog or the like is even lighter than that in FIG. 6A, two or more timings at which the number of response elements N becomes equal to or less than the predetermined threshold Nth may occur, as illustrated in FIG. 6B.

[0050] In this way, even if two or more fall timings Tb are detected due to a thin concentration of fog, etc., reflected light does not occur at a distance greater than the measurement object R, so by determining the light receiving timing based on the latest fall timing Tb, it is possible to suppress erroneous measurements caused by the concentration of fog, etc.

[0051] [Second embodiment] Next, a distance measurement device according to a second embodiment of the present invention will be described with reference to the drawings. The second embodiment differs from the first embodiment mainly in that a passive quench circuit is used in place of an active quench circuit in the light receiving section. Therefore, components that are substantially the same as those in the first embodiment are given the same reference numerals and descriptions thereof will be omitted.

[0052] In the distance measurement device 10 according to this embodiment, a passive quench circuit is employed in the light receiving unit 30 instead of the active quench circuit described above as a circuit for restoring each SPAD to a responsive state. The passive quench circuit is simpler than the active quench circuit and applies a reverse bias voltage Va to each SPAD. The passive quench circuit gradually restores the applied voltage V to the SPAD, which has dropped from Va in response to a breakdown caused by light reception, to Va "by a quenching function using a quench resistor, which is a passive element." Therefore, the applied voltage V to each SPAD gradually recovers immediately after a breakdown (breakdown) caused by light reception. However, even during recovery, there is a high probability of continuous breakdown (breakdown) occurring, especially when the intensity of the received light is strong. In other words, the voltage V applied to the SPAD does not fully recover during the light projection pulse width ΔTv of the light projection pulse (the time during which the light projection pulse is projected), and then the applied voltage begins to recover when the light projection pulse falls (the light projection pulse is no longer received), and the recovery of the applied voltage is then completely completed.

[0053] That is, as shown in Figure 7, the passive quench circuit does not restore the applied voltage while the received light pulse is being received, and functions to return the SPAD to a responsive state within the constant SPAD dead time ΔTd after the light reception time of the received light pulse (the time during which the projected light pulse is projected: the projected light pulse width ΔTv of the projected light pulse) has elapsed.

[0054] In this case, the longer the light projection pulse width ΔTv of the light projection pulse projected from the light projector 20, the later the recovery start timing at which the applied voltage begins to recover, and therefore the longer the time difference between the rising timing Ta2 and the falling timing Tb to be estimated.

[0055] Therefore, in this embodiment, the rise timing Ta2 is estimated by subtracting the SPAD dead time ΔTd and the light projection pulse width ΔTv from the fall timing Tb, thereby suppressing the deviation in the recovery start timing of the applied voltage caused by the light projection pulse width ΔTv that occurs when a passive quench circuit is used.

[0056] For this reason, in the measurement process performed by the measurement unit 40 in this embodiment, when there is an influence of fog or the like, distance measurement is performed using the rise timing Ta2 estimated in consideration of the light projection pulse width ΔTv as described above. Specifically, when it is determined that there is an influence of fog or the like (Yes in S109), the rise timing Ta2 is estimated in the distance calculation process of step S113 as the timing obtained by subtracting the SPAD dead time ΔTd and the light projection pulse width ΔTv from the fall timing Tb. Then, using the rise timing Ta2 estimated in this way as the light reception timing, the distance to the measurement object at the measurement target pixel is calculated according to the time difference ΔT between this rise timing Ta2 and the light projection timing acquired from the control unit 11.

[0057] As described above, in the distance measurement device 10 according to this embodiment, the light receiving unit 30 has a passive quench circuit for restoring the voltage applied to the SPAD, which has dropped from the reverse bias voltage Va in response to a breakdown phenomenon caused by light reception, to the voltage Va when the voltage stops dropping during light reception and light reception ceases. The measurement unit 40 then determines the light reception timing to be the timing obtained by subtracting the SPAD dead time ΔTd and the light projection pulse width ΔTv from the falling timing Tb detected by the signal processing unit 32.

[0058] This suppresses the deviation in the timing at which the recovery of the applied voltage starts, which is caused by the light-projection pulse width ΔTv that occurs when a passive quench circuit is used. Therefore, even when a passive quench circuit is used, it is possible to suppress the deterioration in the estimation accuracy of the rise timing and light-receiving timing that is caused by the deviation in the timing at which the recovery of the applied voltage starts.

[0059] [Third embodiment] Next, a distance measurement device according to a third embodiment of the present invention will be described with reference to the drawings. The third embodiment is different from the first embodiment in that distance measurement is not performed on pixels where the gradient before and after the falling timing of the response waveform is equal to or less than a predetermined gradient threshold. Therefore, components that are substantially the same as those in the first embodiment are denoted by the same reference numerals, and their description will be omitted.

[0060] In an environment where there is fog or the like, if a measurement object R is present at the destination of the light projection pulse, the response waveform is generated with a falling edge that rapidly decreases according to the distance to the measurement object R, as exemplified in the above-mentioned FIG. 4(B). In particular, since the SPAD does not newly respond after receiving the reflected light from the measurement object R, the time it takes for the fall from the maximum value Nmax to complete is the dead time ΔTd of the SPAD. In other words, if the fall occurs from the maximum value Nmax until the dead time ΔTd has elapsed, it can be determined that a measurement object R is present at the destination of the light projection pulse.

[0061] On the other hand, if there is no measurement object at the destination of the light projection pulse, the response waveform is generated so that it maintains the maximum value Nmax to a certain extent and then gradually falls, as shown in Fig. 8. This is because the power of the projected light projection pulse not only attenuates according to the inverse square law of distance, but also attenuates as the light passes through fog.

[0062] In other words, in an environment where fog or the like is present, the slope of the response waveform before and after the timing at which the response value falls below the predetermined threshold value Nth when there is no measurement object at the destination of the light projection pulse will be smaller (gentler) than the slope of the response waveform before and after the timing at which the response value falls below the predetermined threshold value Nth when there is a measurement object.

[0063] 9, in the measurement process performed by the measurement unit 40 in this embodiment, if it is determined that there is an influence of fog or the like (Yes in S109 in FIG. 9), it is determined in the determination process of step S121 whether the slope before and after the timing of the fall of the response waveform is equal to or less than a predetermined slope threshold. Specifically, for example, it is determined whether the slope θ of the response waveform (see FIG. 8) corresponding to the time from the maximum value Nmax to the completion of the fall is equal to or less than a predetermined slope threshold corresponding to the dead time ΔTd of the SPAD.

[0064] If the slope before and after the falling timing of the response waveform is below a predetermined slope threshold (Yes in S121), it is assumed that the response waveform is generated to gradually fall because there is no measurement object at the destination of the light projection pulse, and distance measurement is not performed for that pixel, thereby preventing erroneous measurements due to fog, etc.

[0065] The slope of the response waveform before and after the falling timing is not limited to being determined based on the slope of the response waveform corresponding to the time from the maximum value Nmax until the fall is completed, but may also be determined, for example, based on the slope of the response waveform corresponding to the difference between the number of response elements N a predetermined time before the falling timing Tb and the number of response elements N a predetermined time after the falling timing Tb.

[0066] [Fourth embodiment] Next, a distance measurement device according to a fourth embodiment of the present invention will be described with reference to the drawings. The fourth embodiment differs from the first embodiment mainly in that the light projection pulses are projected so that the light intensity gradually increases. Therefore, the same reference numerals are used to designate components that are substantially the same as those in the first embodiment, and descriptions thereof will be omitted.

[0067] In an active quench circuit, which restores the applied voltage even while light is being received, each SPAD returns to a state where it can respond to reflected light in a shorter time than in a passive quench circuit.Therefore, when strong reflected light is received from fog or other objects at close range during dense fog, there is a risk of an oscillation phenomenon occurring in which the response waveforms of almost all SPADs simultaneously repeatedly exceed a predetermined threshold value Nth.

[0068] For this reason, in this embodiment, the light-projecting unit 20 is controlled to project a light-projection pulse whose light intensity gradually increases. Specifically, for example, as illustrated in Fig. 10(A), the supply current I supplied to each light-projecting element when emitting a light-projection pulse is increased gradually and linearly, and then the supply current I is sharply reduced after a specified time has passed since the start of light projection. This allows the light-projecting unit 20 to project a light-projection pulse whose light intensity gradually increases and then sharply decreases.

[0069] In this way, by controlling the light-projecting unit 20 to project a light-projection pulse whose light intensity gradually increases, the timing at which the response waveform exceeds the predetermined threshold value Nth in each SPAD can be dispersed compared to when a rectangular light-projection pulse is projected, thereby suppressing the above-mentioned oscillation phenomenon.

[0070] The light-projecting unit 20 is not limited to being controlled to project a light-projection pulse in which the light intensity gradually increases by increasing the supply current I in a gentle linear manner, but may also be controlled to project a light-projection pulse in which the light intensity gradually increases by increasing the supply current I in a gentle curve, as illustrated in FIG. 10(B), for example.

[0071] The present invention is not limited to the above-described embodiments, and may be embodied as follows, for example. (1) The present invention is not limited to being adopted in a distance measurement device 10 configured as a SPAD-LiDAR for short-range applications (e.g., a measurement range of approximately 20 m) in which the measurement direction rotates by a predetermined angle for each sampling, but may also be adopted in other distance measurement devices that employ a TOF measurement method.

[0072] (2) The present invention can be applied to a device that measures the distance to an object within a specified measurement area, but is not limited to this. For example, the present invention may be applied to a device that measures the distance to an object that moves relative to the object when installed on a moving body.

[0073] 10...Distance measuring device 11...Control unit 20...Light projecting unit 30…Light receiving section 32...Signal processing unit 40...Measuring unit N: Number of response elements Nth...Threshold Ta1: Detected rising timing Ta2: Estimated rising timing Tb…Fall timing ΔTd: Dead time ΔTv: Light emission pulse width

Claims

1. a light projection unit that projects a predetermined light projection pulse onto a predetermined measurement target area; a light receiving section including a plurality of arrays of a predetermined number of light receiving elements each constituting one pixel, the light receiving section receiving light reflected by an object within the measurement target area from the light projection pulse; a measurement unit that measures distance within the measurement target area based on a time difference between a light projection timing of the light projection unit and a light reception timing determined for each pixel in the light reception unit; A distance measurement device comprising: the light receiving unit includes a signal processing unit that detects, for each pixel, a falling timing at which a response waveform generated in response to a change in the number of light receiving elements that respond to the reflected light, exceeds a predetermined threshold and then falls below the predetermined threshold; The distance measurement device is characterized in that the measurement unit determines the light receiving timing to be a timing obtained by subtracting a dead time of the light receiving element from the falling timing detected by the signal processing unit.

2. the light receiving element is a SPAD (Single Photon Avalanche Diode), the light receiving unit has a passive quench circuit for restoring a voltage applied to the SPAD, which has dropped from a predetermined voltage value in response to a breakdown phenomenon caused by light reception, to the predetermined voltage value when the voltage stops dropping during the light reception and light reception ceases, and The distance measurement device according to claim 1, characterized in that the measurement unit determines the light reception timing to be the timing obtained by subtracting the dead time and the light projection pulse width of the specified light projection pulse from the falling timing detected by the signal processing unit.

3. 2. The distance measurement device according to claim 1, wherein when the signal processing unit detects two or more falling edge timings for one pixel, the measurement unit determines the light receiving timing based on the latest falling edge timing.

4. the signal processing unit detects, for each pixel, a rising timing at which the response waveform exceeds the predetermined threshold together with a falling timing; The distance measurement device of claim 1, characterized in that the measurement unit determines the light receiving timing to be the timing obtained by subtracting the dead time of the light receiving element from the falling timing when the time difference between the rising timing and the timing obtained by subtracting the dead time of the light receiving element from the falling timing is greater than or equal to a predetermined time threshold, and determines the light receiving timing to be the rising timing when the time difference between the rising timing and the timing obtained by subtracting the dead time of the light receiving element from the falling timing is less than the predetermined time threshold.

5. 2. The distance measurement device according to claim 1, wherein the measurement unit does not measure the distance to a pixel for which the gradient of the response waveform before and after the falling timing is equal to or smaller than a predetermined gradient threshold value.

6. the light receiving element is a SPAD (Single Photon Avalanche Diode), the light receiving unit has an active quench circuit for restoring a voltage applied to the SPAD, which has dropped from a predetermined voltage value in response to a breakdown phenomenon caused by light reception, to the predetermined voltage value after a predetermined time even during the light reception; 2. The distance measurement device according to claim 1, wherein the light projecting unit projects the light projection pulses so that the amount of light gradually increases.

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